The big spring auctions in New York get under way this week, with the most stellar lots of Impressionist and modern art hitting the block in evening sales at Christie’s on Tuesday, May 6, and Sotheby’s on Wednesday, May 7, followed by evening sales of post-war and contemporary art at Sotheby’s on May 13, at Christie’s on May 14, and at Phillips, de Pury and Co. on May 15, 2008.